Weight gain “at any time can increase the risk of breast cancer by up to 50 per cent” reported the Daily Mail. There was no difference in risk in “according to how overweight the women...

Weight gain “at any time can increase the risk of breast cancer by up to 50 per cent” reported the Daily Mail. There was no difference in risk in “according to how overweight the women were”, but “any amount of weight gain was important in relation to breast cancer risk”, the newspaper said. This increase in risk can be countered “by losing the excess pounds and returning to a healthy size”, the Daily Mail stated.

This story comes from a large study of postmenopausal women in the US that found that weight gain in adulthood increased the risk of breast cancer in postmenopausal women who were not taking HRT. Women who gained about three to five stones were at about 50% higher risk than women whose adult weight remained stable. Although the newspapers suggest that losing the weight reduces the risk back to the level of a woman with stable weight, the study does not look at the question in this way, and so cannot prove this. Maintaining a healthy weight has many advantages, and this study suggests that this may include helping to keep breast cancer risk at a stable level.

Where did the story come from?

Dr Jiyoung Ahn and colleagues from the US National Cancer Institute, National Institutes of Health (NIH), and AARP (previously known as the American Association of Retired Persons) carried out this research. The study was funded by the NIH and was published in the peer-reviewed medical journal Archives of Internal Medicine.

What kind of scientific study was this?

This was a prospective cohort study: the NIH-AARP Diet and Health Study, looking at the relationship between being overweight and gaining weight as an adult, and breast cancer risk.

This study included 99,039 postmenopausal women aged 50-71 years who did not have a history of cancer when the study began in 1995. Women completed a questionnaire about their medical history and lifestyle when they enrolled, and a more detailed questionnaire about potential risk factors about a year later. This included questions about their weight currently, and at ages 18, 35, and 50 years, and their current height, waist and hip measurements. These data were used to calculate the women’s body mass indices (BMI), and to see whether the women had ever gained or lost substantial amounts of weight in their adult lives. The researchers then followed up these women over the next five years to see if they developed breast or other types of cancer or if they died, using questionnaires, state cancer registries, and national death records.

The researchers looked to see whether women with different weights, body mass indices (BMI), waist to hip ratios, or those who had substantial weight change in adulthood differed in their risk of developing breast cancer. Analyses were adjusted for other factors that might affect risk of breast cancer, including age, family history of breast cancer, education, age at first period, age at menopause, age at first childbirth, how many children the women had, smoking, physical activity, fat intake, alcohol intake, whether they had had their ovaries removed, use of hormone replacement therapy (HRT), and height. Women who currently used HRT and those who didn’t (including those who had never used HRT and women who had used HRT in the past and then stopped) were analysed separately, as HRT was found to affect the relationship between weight and breast cancer risk.

What were the results of the study?

During follow up, 2,111 women developed breast cancer. The researchers found that a higher current BMI, higher BMI at 35 and 50 years old, and greater waist to hip ratio were linked to greater risk of breast cancer in women who were not using HRT, but not those who were using HRT. Conversely, a higher BMI at age 18 was associated with a trend towards lower risk of breast cancer.

Weight gain during adulthood was associated with an increased risk of breast cancer in women who were not currently using HRT. Those not using HRT who had gained 40 kg (about 6 stone) or more between ages 18 and current age had two times the risk of developing breast cancer compared with those who maintained a steady weight (less than 2 kg change). Women not using HRT who had gained between 20-29.9 kg (about three to five stone) after age 18 had about 1.5 times the risk of developing breast cancer compared with the steady-weight group.

What interpretations did the researchers draw from these results?

Researchers concluded that weight gain during adulthood increases the risk of breast cancer in women not using HRT.

What does the NHS Knowledge Service make of this study?

This was a large and relatively well-conducted study. However, there are several limitations.

This study may be subject to recall bias. Researchers asked postmenopausal women to recall what their weight was when they were 18 and 35 years old, which they may not have been able to do accurately. Women also had to report their own height, waist and hip measurements, which may also have lead to inaccuracies. The authors of the study report that women tend to underestimate their weight, and that this has particularly found to be the case with heavier women. If the women in this study had done this, the results could significantly affected.

The significant association between breast cancer and both BMI and weight gain during adulthood was seen only in women who were not currently using HRT. This suggests that the relationship between weight gain and breast cancer risk may be altered by the current use of HRT.

Most of the women included in this study were Caucasian, and these results may not apply to women of different ethnic groups.

This study only looked at risk of developing breast cancer post-menopausally, as women who had had pre-menopausal breast cancer were excluded.

The study did look at whether adult weight loss altered risk of breast cancer, and found that the risk of breast cancer in women who lost weight was no different from those whose weight had remained stable. However, as this analysis did not specifically look at women who were overweight and then lost weight or returned to a healthy weight (i.e. it could include women who were not overweight and lost weight), it cannot be said for certain that women who do this will return their risk to normal levels.

Overall, maintaining a healthy weight has many advantages, and this study suggests that this may well include a reduced risk of breast cancer in women who are not using HRT.

Sir Muir Gray adds...

This is another reason for weight control; women who walk 3000 extra steps daily to control their weight – the best way to prevent weight gain if food intake stays constant – can think of yet another benefit while they do so.